Number of order-7 ribbon tilings for a 7 X n strip.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =120a(5) =720a(6) =5040a(7) =20160a(8) =84000a(9) =364560a(10) =1647744a(11) =7754096a(12) =37979164a(13) =193530835a(14) =883966084a(15) =4052227976a(16) =18669720090a(17) =86583455766a(18) =404839016404